In developing nanocrystals, there are several issues that need to be overcome. For a number of technologically important applications of nanocrystals, such as phosphors for solid state lighting and gain material for optically-pumped cw (continuous wave) lasers, the operating temperature of the nanocrystals is significantly above room temperature. For these applications, the optical excitation power density can range from about 200 (in solid state lighting) to >50,000 (for lasing) W/cm2. Typical type I CdSe-based nanocrystals lose significant quantum efficiency (QE) (about 60%) as the temperature increases from 25° C. to 150° C., as discussed by Zhao et al [ACS Nano 6, 9058 (2012)]. Pradhan et al. [N. Pradhan et al., J. Amer. Chem. Soc. 129, 3339 (2007)] found that Mn-doped ZnSe nanocrystals maintained a reasonable thermal stability up to ˜250° C.
The disadvantages of this latter approach are that the peak emission wavelengths of the nanocrystals are limited by the particular choice of the dopant materials, the spectral widths of the photoluminescence (PL) are typically larger for impurity emission, and the quantum efficiency of these types of nanocrystals is below that of undoped nanocrystals. Type II (confining electrons in one material and holes in another) core-shell CdTe/CdSe nanocrystals were also determined to have reasonable temperature stability up to ˜110° C. [P. Chin et al., J. Amer. Chem. Soc. 129, 14880 (2007)]. Though better than conventional nanocrystals, the reported stability falls short of the above requirements for solid state lighting. Overall, there are no nanocrystals to date which show both high temperature stability and high quantum efficiency.
In addition to thermal stability issues, nanocrystals suffer from high-optical flux saturation as a result of Auger recombination [Y. Park et al., Phys. Rev. Lett. 106, 187401 (2011)] and blinking phenomena [X. Brokmann et al., Phys. Rev. Lett. 90, 120601 (2003)]. In order to obtain optically-pumped lasing of nanocrystal materials, unwanted Auger recombination needs to be highly reduced or shutdown, at least up to an average level of 1 e-h pair per nanocrystal. Type II nanocrystals with thick shells were reported [Y. Park et al., Phys. Rev. Lett. 106, 187401 (2011)] to show substantially less saturation of the PL compared to typical nanocrystals. These results led the authors to claim that they shutdown Auger recombination in these nanocrystals. However, they were only able to attain these results for isolated nanocrystals, since the degree of flux stability varied amongst their nanocrystals.
In another report [D. Oron et al., Phys. Rev. B75, 035330 (2007)], ps-pulsed excitation was used to show that by increasing the electron-hole separation distance in type II nanocrystals, the Auger recombination time could be increased up to ˜2 ns. The problem with this approach is that the oscillator strength of the direct e-h transition decreases with increasing separation distance, thus resulting in greater competition from non-radiative recombination (thus causing a reduction in the overall QE). In addition, though a 2 ns Auger lifetime is longer than typical, it still shorter than desired for type II nanocrystals. Overall, the shutdown of Auger recombination has not been demonstrated in macroscopic nanocrystal samples at room temperature, especially for the very desirable Type I (confining electrons and holes in the same material) core-shell nanocrystals.